Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” Drilling fluid, or “mud,” is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and also carries drill cuttings back to the surface in the annulus between the drill string and the wellbore wall.
For successful oil and gas exploration, it is necessary to have information about the subsurface formations that are penetrated by a wellbore. For example, one aspect of standard formation evaluation relates to the measurements of the formation pressure and formation permeability. These measurements are important for predicting the production capacity and production lifetime of a subsurface formation.
One technique for measuring formation and reservoir fluid properties includes lowering a “wireline” tool into the well to measure formation properties. A wireline tool is a measurement tool that is suspended from a wireline in electrical communication with a control system disposed on the surface. The tool is lowered into a well so that it can measure formation properties at desired depths. A typical wireline tool may include one or more probes that may be pressed against the wellbore wall to establish fluid communication with the formation. This type of wireline tool is often called a “formation tester.” Using the probe(s), a formation tester measures the pressure history of the formation fluids contacted while generating a pressure pulse, which may subsequently be used to determine the formation pressure and formation permeability. The formation tester tool also typically withdraws a sample of the formation fluid that is either subsequently transported to the surface for analysis or analyzed downhole.
In order to use any wireline tool, whether the tool be a resistivity, porosity or formation testing tool, the drill string must be removed from the well so that the tool can be lowered into the well. This is called a “trip”. Further, the wireline tools must be lowered to the zone of interest, commonly at or near the bottom of the wellbore. The combination of removing the drill string and lowering the wireline tool downhole are time-consuming procedures and can take up to several hours, if not days, depending upon the depth of the wellbore. Because of the great expense and rig time required to “trip” the drill pipe and lower the wireline tools down the wellbore, wireline tools are generally used only when the information is absolutely needed or when the drill string is tripped for another reason, such as to change the drill bit or to set casing, etc. Examples of wireline formation testers are described, for example, in U.S. Pat. Nos. 3,934,468; 4,860,581; 4,893,505; 4,936,139; and 5,622,223.
To avoid or minimize the downtime associated with tripping the drill string, another technique for measuring formation properties has been developed in which tools and devices are positioned near the drill bit in a drilling system. Thus, formation measurements are made during the drilling process and the terminology generally used in the art is “MWD” (measurement-while-drilling) and “LWD” (logging-while-drilling).
MWD typically measures the drill bit trajectory as well as wellbore temperature and pressure, while LWD typically measures formation parameters or properties, such as resistivity, porosity, pressure and permeability, and sonic velocity, among others. Real-time data, such as the formation pressure, facilitates making decisions about drilling mud weight and composition, as well as decisions about drilling rate and weight-on-bit, during the drilling process. While LWD and MWD have different meanings to those of ordinary skill in the art, that distinction is not germane to this disclosure, and therefore this disclosure does not distinguish between the two terms.
Formation evaluation, whether during a wireline operation or while drilling, often requires that fluid from the formation be drawn into a downhole tool for testing and/or sampling. Various sampling devices, typically referred to as probes, are extended from the downhole tool to establish fluid communication with the formation surrounding the wellbore and to draw fluid into the downhole tool. A typical probe is a circular element extended from the downhole tool and positioned against the sidewall of the wellbore. Another device used to form a seal with the wellbore sidewall is referred to as a dual packer. With a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the wellbore therebetween. The rings form a seal with the wellbore wall and permit fluid to be drawn into the isolated portion of the wellbore and into an inlet in the downhole tool.
The mudcake lining the wellbore is often useful in assisting the probe and/or dual packers in making a seal with the wellbore wall. Once the seal is made, fluid from the formation is drawn into the downhole tool through an inlet by lowering the pressure in the downhole tool. Examples of probes and/or packers used in downhole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568, and 6,964,301.
Reservoir evaluation can be performed on fluids drawn into the downhole tool while the tool remains downhole. Techniques currently exist for performing various measurements, pretests and/or sample collection of fluids that enter the downhole tool. However, it has been discovered that when the formation fluid passes into the downhole tool, various contaminants, such as wellbore fluids and/or drilling mud primarily in the form of mud filtrate from the “invaded zone” of the formation or through a leaky mudcake layer, may enter the tool with the formation fluids. The invaded zone is the portion of the formation radially beyond the mudcake layer lining the wellbore where mud filtrate has penetrated the formation leaving the (somewhat solid) mudcake layer behind. These mud filtrate contaminates may affect the quality of measurements and/or samples of formation fluids. Moreover, severe levels of contamination may cause costly delays in the wellbore operations by requiring additional time for obtaining test results and/or samples representative of formation fluid. Additionally, such problems may yield false results that are erroneous and/or unusable in field development work. Thus, it is desirable that the formation fluid entering into the downhole tool be sufficiently “clean” or “virgin”. In other words, the formation fluid should have little or no contamination.
A variety of packers are used in wellbores for many types of applications, including fluid sampling applications. In some applications, a straddle packer is employed to isolate a specific region of the wellbore to allow collection of fluid samples. However, straddle packers use a dual packer configuration in which fluids are collected between two separate packers. The dual packer configuration is susceptible to mechanical stresses which limit the expansion ratio and the drawdown pressure differential that can be employed. Other applications rely on a single packer having sample drains positioned to collect well fluid for downhole analysis and/or storage in bottles for later analysis in a lab. The sample drains are bounded by guard drains which are used to collect well fluid in a manner that aids collection of a clean sample through the centrally located sample drains. However, existing designs may have certain limitations in specific sampling applications.